Advances in field emission display technology are disclosed in U.S. Pat. No. 3,755,704, "Field Emission Cathode Structures and Devices Utilizing Such Structures," issued Aug. 28, 1973, to C. A. Spindtetal.; U.S. Pat. No. 4,940,916, "Electron Source with Micropoint Emissive Cathodes and Display Means by Cathodoluminescence Excited by Field Emission Using Said Source," issued Jul. 10, 1990 to Michel Borel et al.; U.S. Pat. No. 5,194,780, "Electron Source with Microtip Emissive Cathodes," issued Mar. 16, 1993 to Robert Meyer; and U.S. Pat. No. 5,225,820, "Microtip Trichromatic Fluorescent Screen," issued Jul. 6, 1993, to Jean-Frederic Clerc. These patents are incorporated by reference into the present application.
One of the technical challenges currently facing researchers in the area of field emission display development relates to the tendency of the electron stream emanating from the electron emitters to disperse at an angle on the order of 30.degree.. Such a dispersion spreads the beam impingent on the luminescent coating of the anode over a relatively wide area, resulting in a image display of poor resolution. Many focusing schemes have been proposed to reduce the dispersion of electrons as they traverse the space between the emitter and collector electrodes. See, for example, U.S. Pat. No. 5,070,282, "An Electron Source of the Field Emission Type," issued Dec. 3, 1991, to B. Epsztein, which discloses a negatively biased control electrode, placed downstream of the extracting electrode, causing the electrons to converge toward the axis of the beam. See also U.S. Pat. No. 5,235,244, "Automatically Collimating Electron Beam Producing Arrangement," issued Aug. 10, 1993, to C. A. Spindt, which discloses a passive dielectric electron beam deflector.
A second technical challenge involves the limitations in gray scale definition as the pixel density of the display screen increases. U.S. Pat. No. 4,857,799, issued Aug. 15, 1989, to C. A. Spindt et al., discloses a row-at-a-time scanning display, in which an entire row of pixels is simultaneously energized, rather than energization of individual pixels. According to this scheme, sequential rows are energized to provide a display frame, as opposed to sequential energization of individual pixels in a raster scan manner. This extends the duty cycle for each panel in order to provide enhanced brightness. Nevertheless, as the size of the display screen increases with an concomitant increase in the number of rows, the dwell time at each row decreases, resulting in a decreased number of gray scale gradations.
A third challenge, somewhat related to the problem raised by the gray scale limitation, is the limitation of image brightness in a scanning display, especially one with a large number of rows. Since the scanning feature allows each row of emitters to emit electrons only during the period that row is being addressed, there must be a very high level of emission and high impact energy at the phosphors in order to provide sufficient luminescent energy to persist, either in the phosphors or in the human optic system or in both, until the next scan period.
A fourth challenge involves the relatively large phosphor area required on the display screen of a field emission display device. The duration of electron emission on each phosphor spot is relatively short dwell time afforded by row-at-a-time scanning. In order for sufficient luminescent energy to be maintained until the next scan period, each phosphor spot must be relatively large in area. However, image contrast is enhanced by having a larger black matrix area surrounding each phosphor spot. Thus, for improved contrast, a reduction in the phosphor spot area is desirable.
The present invention addresses the above-described shortcomings of present field emission displays.